gpio: Only descend into gpio directory when CONFIG_GPIOLIB is set

When CONFIG_GPIOLIB is not set make will still descend into the gpio
directory but nothing will be built. This produces unneeded build
artifacts and messages in addition to slowing the build. Fix this here.

Signed-off-by: Andrew F. Davis <afd@ti.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>